1. Field of the Invention
The present invention concerns an acceleration limiter for a turbojet intended to protect the turbojet from stalling of the compressor or overheating in the turbine.
2. Description of the Prior Art
It is known that the operating conditions of a turbojet, both in stable operation and during acceleration, are governed by the richness, which varies as the ratio C/P, of the flow of fuel C at a characteristic pressure P associated with the air flow passing through the combustion chamber, particularly the delivery pressure P2 of the compressor. In particular, the characteristic ratio C/P must not exceed a set maximum, in order to avoid the risk of the compressor stalling.
For this purpose, the regulator of the fuel flow sent to the injectors is generally equipped with a special device, known as an "acceleration stop," the function of which is illustrated in FIG. 1. This diagram represents the variations in the characteristic ratio C/P according to the rotation speed N of the turbine under various operating conditions. The curve S corresponds to the stable running of the turbojet, corresponding for example to a uniform cruising speed of the airplane on which it is mounted. The broken-line curve B corresponds to the upper limits imposed on the characteristic ratio C/P by the aforementioned acceleration stop. Because of this device, in the case of a sudden acceleration in the flow of fuel C due to a sharp action on the throttle lever, the operating point of the turbojet, starting for example from the stable operating point M, describes a curve such as a, having a rising side a1 that is more or less steep, followed by an arc a2 rising slowly and being slightly below the upper curve B.
However, in the particular case of a turbojet mounted on an airplane, it may occur that regulation of the flow has a characteristic such that, following a sharp action on the throttle level, it induces a flow such that the corresponding curve described by the operating point b has not only a rising side b1 but, before establishment of the arc b3 imposed by the acceleration stop, the operating point exceeds the maximum curve B with a more or less major amplitude b2. In the absence of special provisions, this would result in the compressor stalling or the turbine overheating.
To avoid such drawbacks, the prior art has used one or more hydraulic accumulators connected to the supply circuit of the fuel injectors, these accumulators having the effect of absorbing the excess fuel flow corresponding to the excess b2 on the diagram b in FIG. 1. This known disposition makes it possible to make an alteration in the operating point of the turbojet corresponding essentially to the diagram c in FIG. 2 correspond to a sudden acceleration in the fuel flow; with such an operating diagram, the operating point never passes beyond the maximum curve B imposed by the acceleration stop (both parts c1 and c3 of the curve being connected by a horizontal part c2 which, with the corresponding part of the curve a (FIG. 1), describes an area A essentially proportional to the internal volume of the hydraulic accumulator).
The use of an ordinary hydraulic accumulator with piston and spring is, however, not entirely satisfactory, for the following reason: the axial position x of its piston, with a surface S', is connected to the injection pressure of the fuel Pi that acts on one face of said piston, as well as to the tenseness k of the spring and to the environmental pressure Po, which act on its other face, in the relationship: ##EQU1## However, since the size of the accumulator's cylinder limits the maximum possible variation .DELTA.x in the position of its piston, the accumulator is only usable within the following limits on variation of the injection pressure: ##EQU2##
It follows that the mechanical and size constraints imposed on the realization of such an accumulator make its range of utilization .DELTA.Pi noticeably inferior to the range of necessary variations in the injection pressure Pi, particularly in the case of airplanes able to fly at high altitude.
Other applications of hydraulic accumulators in fuel supply circuits for turbojets are also known and include:
The French Pat. No. 968 718 by ROLLS ROYCE LTD. describes a supply and start device for a gas turbine having an accumulator with piston and spring in which a first chamber is filled with fuel by the supply circuit of the injectors when the turbine is operating steadily. Upon start-up, the gas is under pressure produced by a cartridge push back, in particular, a liquid that fills the second chamber of the accumulator so as to supply the injectors adequately.
The French Pat. No. 1 236 941 by Bristol Siddeley Engines also describes a system to supply fuel to a gas turbine, with a hydraulic accumulator, linked, in particular, to the supply circuit of the post-combustion burners in this turbine.